Thermocouple hot wire anemometer



Jan. 1, 1952 A. R. HOFFMANN EI'AL THERMOCOUPLE HOT WIRE ANEMOMETER TEMPERATURE Filed Nov. 7; 1946 CURRENT O VELOCITY VELOCITY INDI OATOR ANTON R. HOFFMANN WA'LTER J. CR UMP INVENTORS BY mzm ATTORNEY Patented Jan. 1, 1952 a THERMOCOUPLE nor WIRE ANEMOMETER Anton R. Hoffmann, Southport, and Walter J. Crump, Stratford, Conn, assignors to United Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware Application November 7, 1946, Serial No. 708,378

This invention relates to fluid speed indicators.

An object of this invention is to provide an anemometer of the heated-thermocouple type which automatically provides a linear relationship between the temperature differential of the hot and cold junctions of the thermocouple and the velocity of the air stream.

Another object of this invention is to provide an instrument having a heating element for measuring fluid velocity in which current flowing through the heating element is varied as a function of the temperature of the heating element.

It is a further object of this invention to provide a fully automatic anemometer of the heated-thermocouple type which gives a direct velocity reading of the air flow.

of the heated-thermocouple type which has a greatly increased range of sensitivity as compared with conventional anemometers.

Another object is to provide an anemometer which is accurate at low as well as high velocities.

Another object is to provide electronic controls for an anemometer of the heated-thermocouple type.

.Other objects and advantages will be apparent from the specifications and claims and from the accompanying drawing which illustrates what is now considered to be the preferred embodiment of the invention.

In the drawings,

Fig. 1 shows an anemometer according to this invention installed in a helicopter;

Fig. 2 is a series of curves necessary to the understandinging of the theory and operation of the improved anemometer;

Figure 3 is a curve showing the desired operating characteristics;

Figure 4 is a wiring diagram for an anemometer according to this invention.

Referring to the drawings, Figure 1 shows a helicopter l having a fuselage l2 with a sustaining rotor l4 and a torque compensating rotor l6 mounted thereon. The helicopter is provided with an anemometer, constructed according to the present invention and which comprises a pick-up section I! and an indicating section 20.

4 Claims. (01. 73-404) Pick-up section I! comprises a tube or annulus I8 containing a heating element and one or more thermocouples. Tube I8 is supported from fuselage l2 by bracket l9 and has its axis aligned with the direction of forward flight so that air fiows through the tube and. across the heating element at a velocity indicative of the forward velocity of the helicopter relative to the ambient air.

The indicating section 20 comprises an electronic circuit for controlling the supply of current to the heating element in the pick-up section I1 and an air speed indicator which is connected through wiring, to be described hereinafter, to a thermocouple which is heated by the heating element The indicating section is mounted conveniently within the fuselage l2. While the anemometer is shown mounted on a helicopter, it may readily be used in other types of aircraft.

Figure 2 shows the family of curves, I1, I2, Is characteristic of the temperature variations of an electrical resistance having a constant current input, with variations in the velocity of a fluidstream flowing over the resistance. Temperature, as used here, refers to the temperature of the resistance relative to the temperature of the fiuidstream and is conveniently measured by a thermocouple having a hot junction imbedded in the resistance and a cold junction exposed to the temperature of the fluidstream. Referring to the curve I1, it Will be noted that at low air velocities the temperature of a resistance heated thermocouple, having a constant curent input I1, drops rapidly witlra small increase in velocity. But in the upper ranges of velocity, the temperature drop is small for a relatively large increase in velocity. Consequently, if a heating element having a constant current input is positioned in a fluidstream its temperature will not be a satisfactory indication of ve locity over a wide range.

According to this invention the current input to a heating element in a fluidstream' is varied in a novel manner so as to facilitate the measurement of the velocity of the fiuidstream and so as to increase the sensitivity of such measurement over a Wide range of fluid velocities. This is preferably done by controlling the current flow through the heating element-so that the temperature of the element is continuously varied in accordance with changes in velocity as a linear function thereof, as shown by the dashed line 42 in Figure 2. In the preferred embodiment of the 3 invention this is accomplished by utilizing a grid controlled electronic tube, such as a triode, to vary the current flow through the heating element in predetermined relationship to the temperature thereof.

Referring to the dashed line in Figure 2 it will be noted that. the current is increased with increasing velocity up to a maximum at the point 40 where the line contacts the constant current line Is. As the Velocity is increased further the current will decrease. This relationship between the current and the velocity has been plotted in Figure 3. The grid-voltage plate current curve of the electronic tube must, therefore, match the current-velocity curve of Eigure 3 to give the, straight line temperature-velocity curve 42 of Figure 2. Since the characteristic curve of a triode in its saturation range is substantially the same as this current-velocity curve, it is obvious that a tube can be selected or designed which, when Q e ated n its. sethre i h. rang wil g the es red results Th ete. 2 h ehe t e desig the eeleeheh hit ee stehts'e d. 9 eratihs velt se th 9 1 .1 2 flo ou t e hea ed e eiheht s eehtrell d that te pe tu ma he re ulated h heer h h of the h re ty iv ng 5 5 fiflPq lh i i q curve the. e e at v elehe Shown b he l e h 42 (Figure 2).

Figure i shows the heating element, or resistance, 2 2 mounted within the tube i 8 with thermocouples 2,4 and 26 having their hot junctions embedded therein. The resistance is supplied with current by a circuit comprising a battery B, the triode 3B, and the milliammeter 32. The flow of current in thisflrst circuit is controlled by a second circuit including thermocouple 24, a D. C. amplifier, a battery C, and the grid of the triode 35/ Battery (3 maintains a positive bias on the triode grid. The output from the thermocouple amplifier is also connected to the grid but with a polarity which provides a backing voltage opposing the positive battery bias. A decrease in the amplified thermocouple voltage produces an in.- crease in the positive grid bias voltage. An air speed or velocity indicator is contained in a separate circuit actuated by the thermocouple 26. Variable resistances 34, 36 and 38 may be provided to permit the component circuits of the anemometer to be finally adjusted.

When the given voltage is impressed across the resistance 22, the temperature thereof will remain co as um n t e veleeity r ma ns c ns ant When the veleeitr het rom a poin 39 t the oin 40, (F u 2. th tem e a: tu e i the s ahe 2. w ll deereese Th s i cause. a eer pehh ne t m ete hr te e cre s be en he he hhetieh 8 a he cold iuhetieh o th thermocou le 4-, W e cre ses the Beehive tage on, the gr d 9? he trio Th s cha e in grid b as w ll hetease the eur eh flow t reheh the resistehee t? te a va ue de rmined by the voltage on the grid. A, further incr ase i e o ty 9 the hei t 4! l ea l a e responding temperature decrease of the thermoeehrle 3 h eh il netea e fu h th positive voltage of the grid. This increase of the bias on the grid, however, will now decrease the current flow through the resistance 22 since the triode has as re eh the satu at n he n ehdth rid. s h w ett ee hh e 'ehe Whi h uld r iher ly st t the pla e .deeree e ih'eht eh hew wi l MWQEIQWQQQWQ h the rid the a the pe ht 4!- t t refor e i ent tha the temperature f the h rm coup d e min s th 4 grid bias which in turn determines the current flow through the triode and the resistance 22.

Having obtained a straight line relationship between the temperature and velocity, the velocity may be determined by measuring the temperature by means of thermocouple 26 and indicator 20. This indicator may be a voltmeter or an ammeter having a dial suitably graduated in velocity units. Since the temperature voltage relation is linear for a thermocouple, the gradations on the dial will be linear. If desired, the thermocouple 26 may be eliminated and the indicator 20 placed the circuit with thermocouple 24.

While we have shown a preferred embodiment of our invention, it will be understood that modif ed structures and diiferent arrangements of parts could be 'made to accomplish substantially the same function as performed by our device. Thereiore we wish not to be limited in our invention only to that form shown and described hit b the eeehe e the lew he ehhe We eieh h airspee hshe r e t e 3311999 e the eeh h he a he ti e he r ehetr em fiewih ev r se dfheheehte an e ee iiee reei t hee el men h t eh h h'shieam in said tube, means for supplying current to said hea ing lement e th i'me e le h iv to thi ..,..T ?i ,l llh gfifi id hea ng element l mea her eehtre hhe t e eh freh sup lied t ai a n e eme t se tha the. e he d E- F- et he. ther' heeeu le s fihtih 1 a. inear function of the velocity of the airstream fiow h s a d hea in e ement ncludin e e trehi de e eeht 'olle y the Qu h f a th hheehu l and. ha n le tri a h 'eh teristic curve substantially inverse to that of the uncompensated thermocouple output, and means including a se ehd t rmocouple r sp to the temperature of said heating element for indicating the temperature of the latter in terms of eh e ie d 1h ah speed ind t r a helicopter, eh e ee a re ist n e el n da d t be heate h ei reem fl n pest a icopter, means tor supplying heating current to said element, a thermocouple responsive to tempe ature change in said l n a nd means er eht ehihe the h atin current u p d to said element including an electronic device conrell ci hr the vel eee u pu oi a thermoeehhie fer rhe-ihte hing a a gh line relationship between the temperature of said element and the v lee h' ef ai fluid e m. a indicating means for measuring the temperature of said heating element n indicating the speed of said ahstrehht 3. In a thermocouple air speed indicator for a helicopter, a tube for confining a portion of the tree ehetreem fi wihe er said helieepteh an electrical resistance element located in said tube, means fe su pl in hea n h n to s el ment a t e'rm hhle re 'h e to tem ature changes of said element, an air speed indicator responsive to the temperature oi said thermocouple giving a non-:linear indication of the S eed e the eheepter relati e to said a stream, and means tor controlling the heating urrent s plied e sai e e en t CQIQPQll {or the non-linear indications of said air speed indi r ihelu ins a eeeehd her ehel respehswe e tem eratu e eheh s he sa d lemen nti it these tu e t f'ee heh he the cur t t s id, eleme havi chara te isti s inve e t sa d heh heer ind et ehs o said first. mentioned thermocouple, said tube being operated in its saturation range so that the current flow through the heated element is such that its temperature is substantially a linear function of said airstream velocity.

4. In an instrument having a heating element in a fluid stream, means for supplying current to said heating element, a thermocouple responsive to temperature changes in said element, means for controlling the current supplied to said ele-- ment including an electronic device controlled by said thermocouple for maintaining a straight line relationship between the temperature of said element and the velocity of said fluid stream, and means for measuring the temperature of said heating element.

ANTON R. HOFFMANN.

WALTER J. CRUMP.

6 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,156,660 Berg Oct; 12, 1915 1,222,492 Thomas Apr. 10, 1917 1,476,762 Meyer et a1. Dec. 11, 1923 1,691,600 Brush et al. Nov. 13, 1928 1,766,148 Sawyer June 24, 1930 2,314,877 Hall Mar. 30, 1943 2,412,471 Olson Dec. 10; 1946 

